1. Field of the Invention
This invention relates to magnetic resonance imaging (MRI), and more particularly to a method and apparatus to modify MRI processors to detect low-level signals not ordinarily detected and to enhance visualization of surgical devices in an anatomy.
2. Description of the Related Art
Magnetic Resonance Imaging (MRI) is an imaging technique primarily used in medical settings to produce high-quality images of the internal human body. MRI is based on principles of Nuclear Magnetic Resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules.
The human body consists primarily of fat and water. Fat and water have many hydrogen atoms that make the human body approximately 63 percent hydrogen atoms. The nucleus of a hydrogen atom is comprised of a single proton. A property called “spin” is possessed by a single proton in a hydrogen atom. Spin can be thought of as a small magnetic field that causes the nucleus to produce an NMR signal.
During magnetic resonance imaging, an MRI system generates a strong magnetic field. When a target object (containing water molecules or other hydrogenous compounds) is positioned in the field, the field aligns magnetic dipoles of the hydrogen nuclei within the water molecules (and other hydrogen atoms). The magnetic field strength required to so align the magnetic dipoles is typically on the order of one Tesla, but field strengths significantly higher and lower than one Tesla are also used in various applications of MRI. The magnetic field imparts a resonant frequency to the nuclei that is proportional to the field strength. Once aligned by the magnetic field, the magnetic dipoles can be rotated out of alignment by application of radio frequency (RF) energy at the resonant frequency of the system. Electromagnetic radiation is subsequently emitted by the resonating magnetic dipoles (i.e., the protons spinning at their resonance frequency), as they return to alignment with the field. Imaging occurs as a result of detecting such radiation emitted from each of many different regions within the target.
Physicians use catheters and other medical devices (e.g., scalpels, forceps, retractors, biopsy needles, etc.; and implanted devices used for therapy such as sutures, pacemakers, stents, shunts, orthopaedic devices, dental devices, etc.) to treat patients. Various techniques can be used to monitor these devices while internal to the patient to insure proper administration of a medical technique or post treatment for implanted devices.
Devices, such as X-ray machines, are used to monitor the medical device while internal to a patient. MRI systems can also aid doctors in visualizing medical devices while internal to a patient. It is typically desired that the MRI system portray one or more of the instruments while also imaging a selected portion of the patient. For example, it may be desirable to visualize a biopsy needle or catheter inserted in the tissue of the patient. In addition, it is also desirable to have permanently implanted medical devices such as blood filters, stents, or other such implants visualized in the MR image without affecting the image quality of the surrounding tissue structures.
In magnetic resonance therapy (MRT), the presence of both the magnetic and RF fields used in the imaging process place several constraints on each device to be positioned or manipulated near or in the imaging region of an MRI system. One constraint is that the device must be essentially non-ferromagnetic, so that it is not attracted by a magnetic field. This consideration applies to any object that is implanted within a patient being imaged. This is because the magnetic field would subject such an object to undesirable forces and torque's if it were made entirely of ferromagnetic material. Another constraint is that an electrical instrument must be tolerant of the static and pulsed magnetic and RF fields, in the sense that it can function in the presence of these fields. A further constraint is that a metallic implant or other metallic instrument should not be subject to significant induction heating due to the applied RF field. And, the device should not create imaging artifacts that obscure or distort the image of the target.
Because of these typical constraints, devices used in MRT operations have conventionally been made of non-ferromagnetic metal such as titanium, nitinol, some types of stainless steel, aluminum, copper, or brass. Such non-ferromagnetic metal devices, however, have the following undesirable imaging property when imaged together with a patient in an MRI system. The non-ferromagnetic metal devices, just as most non-hydrogenous materials, will be “negatively” imaged by the MRI system as a black void. That is, the device displaces tissue that normally would be imaged. In areas where the patient's tissue structure has a dark gray or black appearance (due to a weak or absent radiation signal from the magnetic dipoles of its water molecules), the negative image (void) created by the device worsens visualization.
Also, metallic, non-ferromagnetic materials (unless they are ultra-thin) may cause unacceptable imaging artifacts when imaged by an MRI system. Such artifacts (which can have the appearance of a halo or glow around the material which would obscure or distort the image of any target material) occur because the presence of the RF field sets up eddy currents in the non-ferromagnetic material, which in turn create inhomogeneities in the magnetic field of the MRI system. In addition, imaging artifacts are caused by incompatibilities in the magnetic susceptibilities of materials that are in the imaging field.
Other devices are made of polymer materials (such as catheters). These devices are hydrogenous and can obscure imaging of a target since the device may be more hydrogenous than the target. Or, the tissue may be more hydrogenous than the device, thus, obscuring the device. Therefore, MRI systems are not capable of detecting all medical devices while internal to a patient, unless these devices have been modified with suitable material for the MRI system to detect the device. By modifying medical devices to contain a means for a typical MRI system to detect the device, the device's structure is modified.
In the case of medical devices, such as catheters (e.g., catheter body balloon, etc.), an MRI system cannot usably detect the device because the device may cause “noise” that distorts the desired image. The device can cause negative imaging (black spots or image faults). Also, some catheter device components have no impact on a desired MRI image. In other words, the device's components do not appear in the image. Therefore, the MRI image will visualize the same with or without the catheter device in place internal to a patient.
Black spots may appear in an MRI image when the central-processor of the MRI system does not recognize the signal from the medical device as valid data. This is due to the central-processor for the MRI system not having sufficient logic to resolve the signal from the medical device. Therefore, the reported image is not precise, and the device appears as black spots in the MRI image, hence the MRI system discards the imprecise data.
In other instances, a medical device does not show up at all in an MRI image. Typically, the central-processor for the MRI system does not recognize the signal from the medical device, so the central-processor sorts the signal as invaluable outlier data, or noise. Thus, the central-processor filters the signal out, to produce a precise anatomic image without the medical device present in the MRI image.
What is needed is a technique and system to accurately display/identify a medical device in an MRI image while that device is internal to a patient. A medical device capable of being accurately displayed/identified in an MRI image while internal to a patient is also needed.